1. Field of the Invention
The present invention relates to an electronic device including a capacitance type touch panel and a control method of an electronic device including a capacitance type touch panel.
2. Description of the Related Art
As a user interface of an electronic device, a capacitance type touch panel is often used. The touch panel type includes various types, that is, a resistive film type, infrared type, and the like, and a capacitance type is normally adopted due to its convenience.
In a capacitance type touch panel a plurality of electrostatic sensor patterns are formed using transparent electrodes made up of ITO (Indium Tin Oxide) or the like. In general, an increase in capacitance value of an electrostatic sensor pattern at a touch position of the user's finger or the like on the touch panel is detected compared to that of a non-operation, and the presence/absence of a touch operation is determined based on the increase in capacitance value.
When a capacitance value in a non-operation state, which value is used as a reference value, is unwantedly changed due to the influence of an ambient temperature or the like, a touch operation cannot often be accurately determined. Japanese Patent Laid-Open No. 2011-198359 describes an arrangement in which a fan is arranged to blow air between a liquid crystal panel and touch panel, and is controlled to cool the liquid crystal panel and touch panel. With this arrangement, the ambient temperature of the touch panel is controlled to be constant, thus preventing erroneous operations. However, addition of the fan used to cool the touch panel results in an increase in device size and that in cost, and the following problem may also be posed.
Each electrostatic sensor (electrostatic sensor pattern) used to measure a capacitance normally uses a cover lens and the like on an operation portion operated by a user. In general, in order to adhere the cover lens and a sheet on which each electrostatic sensor pattern is arranged, they are fixed using an adhesive sheet or the like. Based on the premise that a touch panel operation is a single-finger operation, in case of a relatively small touch panel, when the user touches the entire touch panel by the palm or the like, a portion of that touch operation cannot be determined, and processing for invalidating that operation itself is executed because an operation is made on a region larger than expected.
However, the temperature of the palm of the user is normally higher than that of the touch panel, and when a touch state of the palm on the entire touch panel continues for a long period of time, the entire touch panel is heated by the palm. As a result, dielectric constants of materials such as the cover lens and adhesive in the vicinity of the electrostatic sensor pattern are changed unwantedly. In such case, when the palm is released, the capacitance of the electrostatic sensor pattern is not equal to that before the touch panel is touched by the palm due to the influences of surrounding materials whose dielectric constants are changed, and a capacitance in a non-operation state is undesirably larger than that before the touch panel is touched by the palm. That is, although the palm is released, the touch panel may be erroneously judged as an operation state, and an erroneous operation of the device itself and a user operation disabled state unwantedly continues for a long period of time.
FIG. 10 shows a change in capacitance of an electrostatic sensor pattern when the palm is placed on the touch panel for a long period of time. As indicated by an interval A, when the touch panel is touched by the palm, a capacitance increases abruptly. After that, while the touch panel is being touched by the palm, the dielectric constant around the electrostatic sensor pattern changes due to the temperature of the palm, and the capacitance increases gradually. After that, when the hand is released from the touch panel, the capacitance of the palm decreases abruptly, but it raised by the temperature of the palm does not abruptly return. Therefore, the capacitance assumes a value larger than that before the touch panel is touched by the palm, and returns to an original capacitance value as the temperature of the touch panel decreases gradually. In control for determining the presence of a touch operation when an increase in capacitance beyond a predetermined threshold is detected, before the temperature returns to an original temperature, although no touch operation is actually made, it is erroneously determined that the touch panel is operated.